Siena Cathedral: Baptistry of Saint John: Detail, lower façade of the baptistry

According to the early historians, in 1316 work began on the eastward extension of the choir of Siena Cathedral, with two bays extending over the Vallepiatta. A new baptistery (S Giovanni) was set into the slope beneath to support the new construction. In 1319, probably under the direction of Camaino di Crescentino (fl 1298-1338), the walls and piers of the baptistery were built on the foundations, and in 1320 marble was being brought for the external facing. Work was suspended until 1355, when the baptistery façade was completed up to its present height. The portals of the baptistery are based on a drawing datable to the 1320s (Siena, Mus. Opera Duomo), but three large windows were built instead of the four projected in the drawing. The polychrome marble façade was given a horizontal roof line, but the top section, at the level of the cathedral clerestory, was left in rough stone, and in 1369 the large oculus made for the end of the old choir was transferred to it.